[Not Applicable]
1. Field of the Invention
The present invention relates to a golf ball. More specifically, the present invention relates to a process for manufacturing a core for a golf ball.
2. Description of the Related Art
Currently, most solid golf ball cores contain some form of zinc diacrylate powder in their formulations as a cross-linking co-agent for the main polymer. The polymer may be polybutadiene, natural rubber, polyisoprene, SBR and others. Fillers and peroxide catalysts are also used in manufacturing a golf ball core material. A major problem with zinc diacrylate is the tendency of the zinc diacrylate to adhere to the mechanism mixing components thereby affecting the uniformity of the formulation. This often requires manual removal of the zinc diacrylate from the mechanical components and reintroduction to the formulation. The problem is greater if the compounds of the formulations are mixed in an internal mixer such as a Banbury mixer or Shaw mixer. The zinc diacrylate adheres to the rotators, the sides of the mixing chamber, the ram, and the discharge doors. If the zinc diacrylate adheres to the internal components, removal is difficult, and may result in addition to subsequent batches of mixture. Thus, some batches will have too much zinc diacrylate and some will have too little zinc diacrylate. The amount of zinc diacrylate is critical to the compression and coefficient of restitution of a golf ball. Thus, the amount of zinc diacrylate will affect the performance of a golf ball when hit with a golf club. The prior art has made some attempts to overcome the problem. One example is Tominaga et al., U.S. Pat. No. 4, 561,657, originally filed in Japan in 1983, which discloses coating a zinc diacrylate powder with a high fatty acid such as lauric acid, stearic acid and palmitic acid to prevent sticking and poor dispersion of the zinc diacrylate in a rubber core mixture. However, the prior art has failed to disclose a procedure that has universal application.
The present invention provides a solution to the core processing problems of the prior art. The present invention is able to accomplish this by providing a method that allows for the processing of a smaller amount of the total rubber material to prevent loss of the zinc diacrylate.
One aspect of the present invention is a method for manufacturing a core for a golf ball. The method begins with introducing 50 to 80 weight percent of a total amount of a rubber polymer into a mixing chamber. Next, zinc diacrylate is introduced in an amount of 20 to 40 parts per hundred of the total amount of the rubber polymer, and other materials are also added to the mixing chamber. Next, the zinc diacrylate, the 50 to 80 weight percent of the rubber polymer, and the other materials are mixed within the mixing chamber for 1 to 7 minutes at a temperature of at least 100xc2x0 F. to create a partial volume mixture. Then, the remaining 20 to 50 weight percent of the total amount of the rubber polymer is added to the mixing chamber and mixed to create a full volume mixture.
Another aspect of the present invention is a golf ball core having uniform dispersion of zinc diacrylate therethrough. The golf ball core is manufactured using the manufacturing process of the present invention. The golf ball core has a PGA compression of 50 to 90 and a coefficient of restitution of at least 0.70. The golf ball core may also have a diameter in the range of 1.45 inches to 1.55 inches.
Yet another aspect of the present invention is a golf ball core having a surface Shore D hardness that is no greater than 8 degrees harder than the center Shore D hardness. The golf ball core is manufactured using the manufacturing process of the present invention. The golf ball core has a PGA compression of 50 to 90 and a coefficient of restitution of at least 0.70. The golf ball core may also have a diameter in the range of 1.45 inches to 1.55 inches.